1.0 Executive Summary

This document provides a detailed technical analysis of a reported issue where a Cisco router forwards ICMP (ping) traffic via its default gateway, contrary to the presence of a more specific static route in its Routing Information Base (RIB). The initial diagnostics confirm a discrepancy between the expected routing logic (longest prefix match) and the observed data plane forwarding behavior. This analysis presents a comprehensive methodology to diagnose and resolve the root cause, which is most commonly attributed to an explicit policy overriding the standard destination-based routing lookup.

2.0 Technical Problem Statement

The core issue involves a Cisco router configured with the following relevant static routes:

• Default Route: ip route 0.0.0.0 0.0.0.0 10.10.10.2
• Specific Static Route: ip route 172.16.1.0 255.255.255.0 10.10.10.3

According to the fundamental principle of longest prefix match, any traffic destined for an address within the 172.16.1.0/24 subnet, such as 172.16.1.50, should be forwarded to the next-hop IP address 10.10.10.3.

However, the observed behavior is that a ping or traceroute originating from the router to 172.16.1.50 is incorrectly forwarded to the default gateway’s next-hop, 10.10.10.2. This indicates a deviation from standard destination-based routing.

3.0 Initial Diagnostic Steps Performed

The preliminary troubleshooting correctly focused on verifying the state of the RIB.

• RIB Verification: The command show ip route 172.16.1.50 was executed. The output confirmed that the router has a valid and active entry for the 172.16.1.0/24 network, correctly pointing to 10.10.10.3 as the next hop.

This initial step effectively confirms that the control plane (the RIB) is correctly populated. The anomaly, therefore, lies within the forwarding decision process or the data plane itself. The initial analysis is insufficient as it does not investigate the mechanisms that can legally override the RIB, such as Policy-Based Routing (PBR) or inconsistencies in the Cisco Express Forwarding (CEF) table.

4.0 Comprehensive Troubleshooting and Resolution Methodology

To resolve this forwarding anomaly, we must systematically investigate the components that influence the router’s data plane decisions, moving beyond the RIB.

Step 1: Verify the Forwarding Information Base (FIB)

The RIB is the map, but the FIB (implemented as the CEF table in modern Cisco IOS) is what the router uses for high-speed packet switching. A discrepancy between the two is a primary indicator of a policy override.

• Action: Execute the command show ip cef 172.16.1.50 detail.
• Analysis: This command reveals the exact forwarding decision the data plane will make. Pay close attention to the next hop address listed. Critically, examine the output for any flags or notations such as “pbr policy-routed.” If this flag is present, it is definitive proof that Policy-Based Routing is overriding the destination-based lookup.

Step 2: Investigate for Policy-Based Routing (PBR) Configuration

PBR is the most common cause for this behavior. It allows administrators to define policies using route-maps that classify traffic and dictate a specific forwarding path, bypassing the RIB. Since the problematic traffic is locally generated by the router, we must check for policies applied to local traffic.

• Action 1: Check for a global PBR policy with the command show ip policy. This will display any route-maps applied to locally generated packets.
• Action 2: Check for interface-specific PBR. While less likely to affect a simple ping from the router CLI unless a source is specified, it is a necessary check. Use show running-config | include ip policy route-map to quickly find all instances of PBR application.
• Action 3: If a route-map is identified in the previous steps, analyze its logic with show route-map <map-name>. Examine the match statements (typically referencing an ACL) and the set commands (e.g., set ip next-hop 10.10.10.2). This will reveal why traffic to 172.16.1.50 is being redirected.

Step 3: Control the Source of Router-Generated Traffic

When a router generates traffic, it selects a source IP address, typically from the egress interface. A PBR policy may be configured to match traffic from that specific source. We can isolate this by explicitly defining the source of our test traffic.

• Action: Execute an extended ping: ping 172.16.1.50 source <interface_or_ip_address>.
• Analysis: Use an IP address from a different interface as the source. If the ping then follows the correct static route to 10.10.10.3, it strongly implies the PBR policy is tied to the original source IP address or interface.

Step 4: Remediation

Once the root cause is identified, proceed with remediation.

• If PBR is the cause: The solution is to modify the PBR configuration. Adjust the ACL used in the route-map to be more specific, ensuring it does not unintentionally capture the ICMP test traffic. Alternatively, if the policy is incorrect, modify or remove the set ip next-hop statement from the route-map.
• If CEF is inconsistent (a rare condition): If the show ip cef output is incorrect but no PBR is found, the CEF entry may be stale. Force a refresh for the specific prefix with clear ip cef 172.16.1.50. Use this command with caution in a production environment.

5.0 Conclusion

The reported forwarding behavior, while appearing anomalous, is characteristic of an explicit policy overriding the standard routing table. The discrepancy between the RIB (show ip route) and actual packet path (traceroute) is the key symptom. A thorough investigation focusing on the CEF table and any configured Policy-Based Routing is the most effective path to diagnosis. By verifying the CEF entry and auditing route-maps, the precise rule causing the deviation can be identified and corrected, restoring predictable traffic flow aligned with network design intentions.